Los Angeles County sheriff's officials say Daniel Holbert, 48, met his victim at the G Train Fitness Gym in North Hollywood in March.
After Holbert treated the victim for pain with several massage therapy sessions at the gym, he suggested future therapy sessions at either his home or hers, which he said would provide more privacy.
The victim agreed, since Holbert had been acting appropriately during all of their previous sessions, authorities said.
On March 12, Holbert arrived at the victim's Santa Clarita home, where he sexually assaulted her during the course of the therapy session, sheriff's officials said.
The District Attorney's Office filed one count of sexual penetration by false representation against Holbert, who was arrested on Tuesday.
Detectives believe there are other women who have been sexually assaulted by Holbert during massage therapy sessions. If you have any information regarding this investigation, you're urged to contact the Los Angeles County Sheriff's Department's Special Victims Bureau at (877) 710-5273.